(1) Field of the Invention
The present invention relates to a lithium secondary battery easy to produce and superior in operational stability and reliability.
(2) Description of Related Art
In recent years, lithium secondary battery has been found practical application as a secondary battery small in size and high in energy density which can function as an electric source of electronic appliances such as portable communication appliance, notebook type personal computer and the like, which are becoming increasingly smaller. Further, in a situation where resource saving and energy saving are internationally drawing people""s attention for the protection of global environment, lithium secondary battery fur use as a battery for driving the motor of electric vehicle or hybrid electric vehicle is being developed in the automobile industry. In the electric power industry, lithium secondary battery is expected as an equipment for night storage of electricity, for the effective use of electricity, and attention is being focussed on the early development of a practical large-capacity lithium secondary battery suitable for such application.
Lithium secondary battery uses a lithium transition metal compound oxide or the like as the positive electrode active substance and a carbon material such as hard carbon, graphite or the like as the negative electrode active substance. During charging, the lithium ion in the positive electrode active substance moves into the negative electrode active substance via an electrolytic solution which is a solution of lithium ion electrolyte in organic solvent, and is captured; during discharging, a reverse battery reaction takes place.
Thus, lithium secondary battery is a chargeable and dischargeable secondary battery. Since lithium secondary battery has a high voltage and a high energy density as compared with conventional secondary batteries such as lead-acid battery and the like, safety mechanisms are employed therein in order to avoid the troubles which may occur owing to abnormalities during charging and discharging. Lithium secondary battery needs to have, for example, a pressure-releasing valve as a safety mechanism for prevention of the bursting of battery which occurs owing to the increase in battery temperature caused by various reasons such as over discharging (due to the short circuiting of output terminal), rapid or excessive charging (due to the failure of charger), application of reverse-direction voltage (due to the mistake of operator) and the like.
In JP-A-10-340717 is disclosed, as an example of the pressure-releasing valve, a safety valve constituted by closing a pressure-releasing hole formed in the lid of battery, with a rectangular thin plate having grooves (these grooves are broken when the internal pressure of battery increases). Also in JP-A-9-92338 is disclosed a pressure-releasing valve constituted by fitting a valve pressed by a spring, to the lid of a battery to seal the battery (when the internal pressure of the battery increases, the valve pushes the spring to release the internal pressure).
The rectangular thin plate disclosed in JP-A-10-340717 is fitted to the lid of battery by laser welding. Therefore, the welding of the rectangular thin plate has problems in that a high equipment cost is required, the welding operation requires a skill, and uniform welding is difficult. The pressure-releasing valve disclosed in JP-A-9-92338 is provided in a state projecting from the end of battery; therefore, the workability of connecting a plurality of batteries in series or parallel is low, the connected batteries are presumably difficult to pack, and the large size and complicated internal structure of the pressure-releasing valve are considered to pose problems in weight and cost.
When a pressure-releasing valve having any one of the above-mentioned structures is used in a secondary battery, it is absolutely necessary, in order for the pressure-releasing valve to function as such, that the case of the battery is sealed tightly. Accordingly, it is necessary that at, for example, the area of the lid to which the pressure-releasing valve is fitted or the area at which the lid and the battery case are welded to each other, the associated members are tightly welded or sealed to each other.
As mentioned previously, in the lithium secondary batteries disclosed in JP-A-10-340717 and JP-A-9-92338, a metal pipe or the like is used as the battery case and the two ends thereof are sealed with a metal-made lid by laser welding; and there remain problems in equipment cost, production cost and workability.
In JP-A-10-241645 is disclosed a method of tight sealing by caulking of a gasket. In JP-A-7-130341 is disclosed a method of tight sealing by caulking of a gasket containing a propylene-ethylene copolymer.
In these tight sealing methods using a gasket, however, caulking is conducted without controlling the load of caulking or the deformation of gasket; therefore, the gasket is deformed in the plastic deformation range or the spring back of the metal (to which caulking is made) is not sufficiently absorbed; consequently, no sufficient a real pressure is obtainable and the leakage of non-aqueous electrolytic solution may occur.
In view of the above-mentioned problems of the prior art and the needed improvement therefor, the present invention aims at providing a lithium secondary battery of low cost wherein a pressure-releasing valve of simple structure has been fitted by a simple method while the reliability is retained.
According to the present invention, there is provided a lithium secondary battery comprising:
an electrode body obtained by winding or laminating a positive electrode and a negative electrode via a separator,
a non-aqueous electrolytic solution, and
a battery case accommodating the electrode body and the non-aqueous electrolytic solution,
wherein two or more members are adhered to each other with a resin or pressure-welded to each other via an elastomer, or a resin is applied to or in the vicinity of the part where two or more members are pressure-welded, and thereby the battery case has a tightly sealed part.
In the lithium secondary battery of the present invention, the resin used is preferably an adhesive composed mainly of a polyimide, or a polyolefin type adhesive. A resin having good corrosion resistance to electrolytic solution and a high adhesive function is preferred. The members adhered or sealed using such a resin include a lid for battery case and a metal foil, which constitute a pressure-releasing valve. When there is employed a pressure-releasing valve constituted by closing a pressure-releasing hole formed in a lid of battery case, with a metal foil and when the metal foil is adhered to the lid using the above resin to close the pressure-releasing hole, the formation of the pressure-releasing valve is easy and simple and can be made for the lid per se. When the fixation of the metal foil is made using a resin and further employing caulking (the metal foil is pressure-welded), better sealing is secured.
As other form of the pressure-releasing valve, there can be mentioned a pressure-releasing valve constituted by closing a pressure-releasing hole formed in a lid of battery case, with a metal foil by means of bending a projection of the lid formed in the vicinity of the pressure-releasing hole, to caulk the metal foil via a spacer. As the spacer, a metal material having a Young""s modulus of 170 GPa or more is used preferably. Use of a spacer of ring shape having a curvature at the inner edge is preferred because it can prevent the damage of the metal foil caused by contact with the spacer and can keep the properties of the pressure-releasing valve at a required level. The radius of the curvature at the inner edge of the spacer is preferably 30 xcexcm or more and xc2xd or less of the spacer thickness.
In such a pressure-releasing valve using a metal foil, it is preferred to place, in the pressure-releasing hole of a battery lid, the metal foil and a resin film having corrosion resistance to electrolytic solution, in two layers so that the resin film faces the interior of battery, because the corrosion of the metal foil can be prevented more reliably. The metal foil and the resin film need not be adhered to each other, but may be adhered with an adhesive. When they are adhered, designing of battery need be made so that the pressure releasability of pressure-releasing valve is not changed. As the resin film, there can be used a film made of a polyethylene, a polypropylene, a polyimide and a fluororesin.
The metal foil is preferably one composed mainly of Al, Cu or Ni. A metal foil of higher purity is preferred because it has good corrosion resistance to non-aqueous electrolytic solution. The metal foil is preferably coated with a fluororesin. Of course, the metal foil may be made of an alloy of the above-mentioned metals.
When two members are pressure-welded via a metal foil to obtain tight sealing, there is, for example, a case that, in addition to the metal foil, an elastomer is interposed between the members. There is also a case that two members are pressure-welded via an elastomer alone to obtain tight sealing. In such a case, it is preferred to conduct pressure welding so that the deformation of the elastomer in the load direction becomes larger than the spring-back amount of the caulked portion and the stress applied to the elastomer becomes not smaller than 980 kPa and not larger than a level at which the retention of elasticity of the elastomer becomes 95% or more. Thereby, tight sealing is secured and the leakage of non-aqueous electrolytic solution can be prevented.
The elastomer used at the sealing part of the present lithium secondary battery is preferably an elastomer processed into a predetermined dimension, i.e. a packing. As the specific material for the elastomer, there can be mentioned an ethylene-propylene rubber, a polyethylene, a polypropylene and a fluororesin. It is preferred to provide, for at least one of the members pressure-welded to each other via an elastomer, a stopper for controlling the deformation of the elastomer. The stopper can be provided preferably in the pressure-releasing hole formed in the lid of the battery.
In the present lithium secondary battery, since the resin used has an excellent corrosion resistance, reliability is secured even when a non-aqueous electrolytic solution containing a carbonic acid ester type organic solvent is used. Also in the present lithium secondary battery, use of a lithium manganese oxide spinel composed mainly of Li and Mn, having a cubic spinel structure, as the positive electrode active substance is preferred because an improvement in battery properties is obtained. The constitution employed in the present lithium secondary battery is preferably applicable to a battery having a capacity of 2 Ah or more. The resulting battery can be used preferably as an electric source battery for the motor of electric vehicle or hybrid electric vehicle.